1. Field of the Invention
The present invention relates in general to a dielectric ceramic body, a method of producing the dielectric ceramic body, and a circuit board which uses the dielectric ceramic body. More particularly, the present invention is concerned with a dielectric ceramic body suitable for high-frequency circuit boards such as microwave integrated circuit (MIC) boards, a method of producing such a dielectric ceramic body, and such a microwave circuit board using a dielectric ceramic body.
2. Discussion of the Prior Art
Keeping pace with recent developments of various microwave devices such as pocket or portable telephones, there has been a growing demand for small-sized high-performance MIC boards used in such devices. In particular, a dielectric ceramic used to form a ceramic substrate for such MIC boards is required to have the following physical and structural features:
(1) The dielectric ceramic should have a sufficiently high specific dielectric constant (.epsilon.r), to reduce the size of a resonance circuit, an inductance element or inductor and a capacitor of a MIC board, when the board is operated at a given frequency.
(2) The dielectric ceramic should have a sufficiently high Q value, to increase the Q value of the resonance circuit and inducatance element of the MIC board.
(3) The temperature coefficient of the dielectric constant of the dielectric ceramic, and the temperature coefficient of the resonance frequency of the resonance circuit should be sufficiently low.
(4) The dielectric ceramic should permit conductor patterns to be formed in a multi-layered or multi-level structure.
(5) Further, an electrically conductive composition used to form the conductor patterns should have a sufficiently low resistivity. In this respect, it is noted that an increase in the resistivity of the conductor patterns reduces the Q value of the resonance circuit and inducatance element of the MIC board, thereby resulting in an increase in transmission loss of the conductors.
There have been known various dielectric ceramic compositions for the ceramic substrates for such MIC boards, such as: a dielectric ceramic composition as disclosed in laid-open Publication No. 58-20905 of examined Japanese Patent Application, whose major components consist of BaO and TiO.sub.2, a portion of which may be substituted by another element or other elements; a dielectric ceramic composition as disclosed in laid-open Publication No. 59-23048 of examined Japanese Patent Application, which has a composite perovskite structure such Ba(Mg.sub.166 Ta.sub.170)O.sub.3 ; a dielectric ceramic composition as disclosed in laid-open Publication No. 54-35678 of examined Japanese Patent Application, whose major components consist of TiO.sub.2, ZrO.sub.2 and SnO.sub.2, a portion of which may be substituted by another element or other elements; and a dielectric ceramic composition as disclosed in laid-open Publication No. 56-26321, whose major components consist of BaO, TiO.sub.2 and RE.sub.2 O.sub.3 (RE: rare earth element).
While the known dielectric ceramic compositions as described above have a relatively high dielectric constant, circuit boards using ceramic substrates formed of these ceramic compositions must use Pt, Pd or other electrically conductive materials which have a relatively high resistivity, where the internal conductor patterns are formed in a multi-level structure within the substrates. Namely, the firing or sintering temperatures of those dielectric ceramic compositions are so high that Ag, Cu, Au or other conductive materials having a sufficiently low resistivity cannot be used for the internal multi-level conductor patterns.
For the ceramic substrates, there are also known inexpensive ceramic compositions such as alumina, steatite and forsterite. However, these ceramic compositions have excessively high temperature coefficients, and undesirably limit the choice of the conductive material for the internal multi-level conductor patterns, to Pt, Pd, W, Mo or other conductors having high resistivity values.